282 GRAZING INDICATORS. 



corrugata, and A. pabularis. These are followed by Atriplex confertifolia and 

 Grayia, which furnish forage of much better quality and larger amount, and 

 these are finally invaded by Artemisia tridentata to form the mixed or pure 

 grazing type so characteristic of the Great Basin and its outlying regions. 

 In the bad lands of the Painted Desert in northern Arizona, the general course 

 of the sere is much the same, but the grasses replace Atriplex. The normal 

 sequence in the subclimax stages is the replacement of Sporobolus airoides by 

 Hilaria jamesii, and this by Bouteloua, often with Muhlenbergia also. The 

 course of development in the halophytic bad lands is essentially a part of the 

 widespread succession in saline basins, except that the latter often begins in 

 water. Shantz (1916:233) has indicated the course of the succession in 

 detail, and it must suffice to point out that the first important indicators of 

 grazing are usually scrub dominants, Sarcobatus and Atriplex. Some of the 

 playas of the Southwest are intensely saline, and show essentially the same 

 communities, but the majority are secondary in nature and belong to the 

 subsere. 



Subsere communities as indicators. — Subseres are developed in secondary 

 areas, such as are regularly produced by fire or cultivation. They occur also 

 in other bare areas in which the disturbance is not sufficient to destroy the 

 soil or to make extreme conditions for ecesis. They are a constant feature of 

 overgrazing and a normal consequence of the presence and activity of man. 

 They are usually local and of small extent, but in the case of fire they may 

 occupy hundreds of square miles. The successional movement is normally 

 rapid, but its progress may be slowed or stopped by the recurrence of the 

 disturbing agency. When this is the case, the area concerned may be held 

 more or less permanently in a subclimax or other serai stage. The most im- 

 portant and extensive subseral communities are those due to fire. The con- 

 sequences of overgrazing often cover great stretches, but the actual com- 

 munities change more or less, or they are much interrupted. Those due to 

 cultivation are usually confined to fields, though many of the dominants 

 become extended to roadsides, and some even enter the natural vegetation. 

 While they often have grazing value, it is incidental and temporary and their 

 chief value lies in connection with utilization as supplementary forage crops, 

 as already indicated for Salsola, Helianthus, Melilotus, and others (plate 68). 



Certain grasses, such as Poa, Avena, and Bromus, have become widespread 

 dominants as a consequence of the combined action of two or more agencies. 

 In the case of Avena and Bromus, the species concerned, A.fatua, B. tectorum, 

 B. rubens, etc., are annuals which have replaced the native dominants as a 

 general result of the combined effect of overgrazing and fire. As annual 

 grasses, these should have a low grazing value, but this is much less true of 

 Avena than Bromus, owing largely to the difference in size and habit. Even 

 Avena is less valuable than the native perennial grasses which they usually 

 replace, and this suggests the desirability of taking advantage of the principles 

 of succession to restore the original community where it has not been com- 

 pletely destroyed. Poa pratensis as a perennial grass of meadows has practi- 

 cally the same ecological habits and grazing value as the prairie dominants 

 which it replaces. Its rapid spread in the valleys and ravines of the true 

 prairies seems to have been the result of a certain amount of disturbance, but 



